Generally, in the downlink closed-loop multiple-input/multiple-output (MIMO) technologies, a base station (BS) should know transmission channel information. For this purpose, the base station needs to support such that mobile stations (MS) can measure downlink channels by transmission antennas of the base station, and the mobile stations need to notify the base station of the measured channels.
At this time, in a closed-loop multiuser-MIMO (MU-MIMO) that requires the transmission channel information, better performance can be achieved when a transmitter acquires full channel status information (CSI) than when it acquires partial CSI of the full channel status information.
However, in order that all of the mobile stations measure the downlink channels by transmission antennas of the base station and the measurement results are fed back to all the base station, an excessive overhead is needed in the downlink and uplink. Accordingly, a partial CSI-based closed-loop MU-MIMO that uses the partial CSI has been widely used while suffering from performance degradation.
In the partial CSI-based closed-loop MU-MIMO, each of the mobile stations feeds back the index of a beam, which is most suitable therefor, among the beams to be used by the base station and a signal-to-noise ratio (SNR) to the base station, instead of the full CSI.
In case of the space division multiple access (SDMA), in which the same spectrum resource is repetitively allocated to multiple mobile stations, the feedback is more complicated. This is because, for SDMA scheduling, the base station should know the interference between beams used by difference mobile stations to which the same spectrum resource is allocated. Particularly, in a wideband communication-based OFDM, since the channel characteristics are different by subcarriers, a more excessive overhead is needed.
The CSI measurement required for the closed-loop MU-MIMO and the feedback of the measured CSI according to the prior art will now be described.
First, a method that uses a MIMO midamble adopted by IEEE 802.16e or the like is known. This method has been most widely used.
According to this method, as shown in FIG. 1, the base station transmits a downlink OFDM symbol with the MIMO midamble, such that the mobile station can determine feedback parameters such as the beam index and the like. FIG. 1 is a diagram illustrating the structure of a MIMO midamble in the related art.
When the subcarrier index used in the OFDM is “i” (where i=0, . . . , N−1), and the number of transmission antennas is 4, the i-th subcarrier is used to transmit a pilot for channel estimation related to the k-th transmission antenna according to Equation 1.i%4=k (where k=0, 1, 2, 3)  [Equation 1]
The mobile station measures the channels by transmission antennas using the pilots, which are allocated by transmission antennas, acquires the entire subcarrier channels by interpolation, and determines the beam index and the like using information about the acquired subcarrier channels.
According to the above-described method, each of the mobile stations acquires the full CSI, but feeds back partial CSI of the full CSI to the base station. In this case, however, there is a problem in that one OFDM symbol is always needed.
Second, a method for channel estimation based on an impulse signal is known.
This method is for channel estimation for demodulation, not the MIMO. It has been known that it is practical in view of the pilot overhead and under fast mobile communication.
In addition, this method is an OFDM system that does not use a cyclic prefix (CP). As shown in FIG. 2, a transmitter inserts null periods, between effective OFDM symbols 210, 211, and 212, then correspondingly inserts impulse signals 213 and 214 at the centers of the null periods, and subsequently transmits the effective OFDM symbols 210, 211, and 212 and the impulse signals 213 and 214. At this time, each of null periods is twice as large as the length of a channel impulse response. FIG. 2 is a diagram illustrating a method for channel estimation based on an impulse signal in the prior art.
As shown in FIG. 2, due to the multipath transmission effect, the effective OFDM symbols 210, 211, and 212 are received in the form of received effective OFDM symbols 251, 252, and 253 after having passed through the channel. In addition, the impulse signals 213 and 214 are received in the forms of first impulse received signals 260, 261, and 262 and second impulse received signals 263, 264, and 265, respectively.
According to this method, since impulse signal transmission power for transmitting the impulse signals 213 and 214 is set to be larger than effective OFDM symbol transmission power for transmitting the effective OFDM symbols 210, 211, and 212, the channel impulse response is easily measured by the mobile station. In addition, the mobile station may acquire the channel estimation value with respect to each subcarrier by transforming on the channel impulse response by fast Fourier transform (FFT).
According to the above-described method, however, when the impulse signals (213 and 214 shown in FIG. 2) are transmitted with power that is larger than that of the effective OFDM symbols (210, 211, and 212 shown in FIG. 2) for data transmission, the value of a peak-to-average power ratio (PAPR) is increased, and a large capacity power amplifier is needed, which causes an increase in cost.
Most of the wireless communication systems use a band pass filter to follow the regulation of electrical radiation, which is defines so as to not interfere with other wireless communication systems or neighboring radio channels. The impulse signal transmission power is distributed in all of the bands.
Thereby, when the band pass filter is not used, the impulse signals strongly interfere with other wireless communication systems and neighboring communication channels. Meanwhile, when the band pass filter is used, power distributed in the bands other than the channel band may be lost, and the impulse signals may be distorted. Accordingly, it is actually difficult for the mobile station to measure the channel impulse response using the impulse signals.
Due to the two above-described reasons, it is difficult to apply the method for channel estimation using the impulse signals to a real system.
As such, in the related art, at the beginning of the introduction of the OFDM technology, a technique that inserts a null guard interval (GI) between the effective OFDM symbols in order to eliminate inter-symbol interference (ISI) has been used. In this case, however, there is a difficulty in the OFDM symbol timing recovery, and performance degradation is caused by the inter-carrier interference (ICI) in the OFDM symbol.